726.—Examination and Adjustment of Chains.—Respectable makers send out chains tested to within half of one of the small links of standard, that is, within a quarter of an inch; but in use this error may increase either by the bending of the long links of the chain, when it becomes shorter, or in the more general case of friction from wear and from strain, by which it becomes longer. In London, standards are fixed upon the pavement in Trafalgar Square and at the Guildhall. These standards are also fixed at many municipal town halls. Surveyors very commonly lay down a standard on the pavement, or by pegs on a level gravel path. Where a peg is used it should be driven home nearly to the surface. It should if possible be made of a piece of heart of oak 12 inches long and about 2½ inches square. The standard length, which may be set off by a standard chain or new steel tape, should be from a saw-cut across the centre of one peg to a similar cut on the other. It is well also to have the centre space (50 links) indicated by a smaller peg.
727.—The Chain to be Adjusted should be first examined and its long links set straight by means of a hammer on a flat, hard stone or anvil, after which the error will be, if it has been much used, that it is too long. It should be then laid in direct line on the standard just described, and stretched lightly with a pull of about 7 lbs., and then left to rest. Assuming it too long, the centre of the chain should be observed to ascertain which half is of the greater length, then short links should be taken out at distributed distances, if more than one be required, by twisting the link open in a vice, and opening and closing another link to restore the chain.
Fig. 336.—Stanley's vice for adjusting and repairing land chains.
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728.—Chain Vice.—The links of steel chains can seldom be twisted open without breaking, and broken links cannot be restored by steel links. Iron links answer, but they are very stiff to twist open. Generally it will be found best for professional men to repair the chain with spare brass links. These wear very well. Where a smith is near with his vice and a light hammer the links are readily opened. It often occurs in open districts and abroad that no smith's shop is to be found. To meet these cases the author has constructed a special vice, as shown Fig. 336. This vice is let into a piece of hard wood—an old oak post answers admirably. In stone districts it is perhaps better to let it into a stone and fix it by pouring hot lead round it. The part B is used for an anvil for straightening the links. The vice V holds the link edgewise very firmly by bringing up the slide J by means of the screw S. The link may then be knocked open by the pane end of a light hammer. The link is closed again in the same manner. If the vice be left out of doors the screw should be well greased and the whole covered with a leaden cover. The weight of the vice is about 6 lbs. It is made of cast iron with chilled face, or the jaws are faced with steel.
729.—Opening and Closing the Chain for Use.—The chain is most readily unfolded by taking the two handles in the hand and walking away from it as it lies on the ground. It is most convenient to place it about 45°, and half a chain length from the first station, each chainman taking a handle and moving to his position. The only danger in undoing a chain is from two chainmen taking one handle each and walking in opposite directions, in which case, if there happens to form a kink, the opposite movement of the two men will probably stretch or break the chain. In closing the chain it is taken by the middle links and folded up two links at a time till the handles are reached. If the links be placed consecutively in position round the axis formed by the first links, it may be folded up very compactly in a twisted form ready for the strap, by which it is carried, to be passed round it.
730.—Chaining is performed by two chainmen, termed the leader and follower. The follower, having pressed a stake into the ground for a starting point, then places the centre of the outside of the handle of the chain against it. The leader takes ten arrows in his right hand and one handle of the chain in his left, and walks directly towards a point which is to be the termination of the measurement, stopping at nearly the length of the chain, examining the chain to see that it is straight. He then places an arrow lightly outside the centre of his handle. The follower looks over this arrow to the distant station to see whether it is in direct line. If it be not so, he waves his right or left hand once, twice, or thrice for 1, 2, or 3 inches for movement to right or left. The follower picks up the arrows consecutively as left by the leader, and when he has the ten, 10 chains have been measured, which is then recorded in the field-book, or earlier than the ten if a shorter distance or object completes the measurement. It is most important to observe that if an arrow be taken for the first station, the follower having ten counts nine only for the first ten. To prevent accident it is therefore safer to start from a stake or other landmark, not one of the arrows. Some surveyors advise eleven arrows. If eleven be used, one should be distinctly marked from the rest so as never to be counted. This may be done by omitting the red webbing tie, or using a green tie for the odd arrow. The French always make the drop arrow the eleventh arrow, which is never counted in direct chaining.
Fig. 337.—Caink's rule for correcting inclines.